Giardia lamblia is the most common disease-causing parasite in the United States responsible for an estimated 3 million cases a year. The surface proteins of Giardia change in the test tube as well as in infections in animals and man and is likely important for the survival of the parasite in humans and animals. In the past we have studied the nature of these changing surface proteins( variant specific-surface proteins or VSPs), important regions of these proteins, how they are transported to the surface of the organism and important structural regions. The molecular mechanisms used to change from one VSP to another are not known. Using a transfection system previous developed in this laboratory, we studied if a VSP could be appropriately turned on and off when controlled by its own upstream and downstream DNA sequences similar to other Giardia proteins when placed in the organism in its own piece of DNA. Although the VSP was made and placed on the surface and appeared to come and go similar to true antigenic variation, additional studies showed that transcription or how these proteins were made was abnormal and therefore could not be used to study antigenic variation. Another method was successful. We put a small antigenic marker in a vsp gene so that when that particular vsp was made we would know because we could detect the antigen( as well as the vsp it is in) on the surface of the organism when the VSP was being made. Then we placed the VSP, its marker, and its own upstream and downstream DNA into the genome or DNA of the same type of Giardia that made the VSP originally. Studies showed that the VSP was turned on and off exactly like the non-manipulated vsp. Our ability to mark a specific vsp gene that undergoes antigenic variation allows us to determine changes in that identical piece of DNA associated with the vsp being turned on or turned off.